The specific aim of this study is to define the emerging role of Magnetic Resonance Imaging (MRI) in radiation therapy treatment planning (RTTP) by: 1) identifying requirements for its effective use through refinement of its integration into a fully 3- dimensional interactive computerized treatment planning system, and 2) studying its relative usefulness through evaluation of the effect its addition has on state of the art CT-assisted treatment plans. The results of this study will lead to the determination of the clinical utility and cost-effectiveness of MRI in RTTP. The first part of the study will further refine the tools and methodology necessary in order to quantitatively utilize the diagnostic information available from MRI. Proposed enhancements involve integration of CT and MRI data such as: automated quality assurance, definition and correction for 3-D distortions, display and refined spatial matching of images, visual and quantitative detection of tumor extents; all of which have additional meaning and utility beyond RTTP. Regular testing and (automated) interpretation should greatly improve the reliability and ultimate quality of all CT and MRI studies. Similarly, the ability to characterize tumors and their extent optimally will aid diagnosis and selection of appropriate therapy. True 3-D matching and unwarping of image data may eventually aid in the subtraction and analysis of follow-up or periodic imaging for detection of cancers and other abnormalities. The potential value of MRI for RTTP will be evaluated in a series of patients for four specific sites (brain, head and neck, thorax, prostate) plus a miscellaneous category. These sites present a comprehensive testing ground for the techniques and methodology of MRI-assisted RTTP. Each patient's treatment will be planned by a team consisting of a radiotherapist, a physicist, and a dosimetrist, with input from a radiologist, using all available standard diagnostic information, including x-ray localization and CT scanning. This will be followed by a second planning session utilizing all the above information with the addition of an MRI data set, and an independent treatment plan will be generated. The differences between CT and CT+ MRI designated volumes and treatment plans will be compared and analyzed. In each site, the degree to which the addition of MRI changes the target volume and the planned treatment technique and the clinical significance of those differences, will be assessed. Those data will be categorized and correlated with reasons for plan changes, thus providing a pilot study of MRI's clinical utility in RTTP.